This invention relates to the control of grain structure in aluminum-silicon alloys. More specifically, it relates to master compositions for modifying the aluminum-silicon eutectic component of aluminum-silicon eutectic and hypo-eutectic casting alloys.
The desirability of adding strontium to other metals in order to alter or improve the properties of the latter has long been recognized in the metallurgical arts. Thus, it is known, for example, as described in U.S. Pat. No. 3,374,086, that strontium, in the form of various ferroalloys, is an effective "innoculant" for the purpose of imparting ductility to cast iron. As another example, the addition of heavy metals, such as the transition elements, to molten aluminum baths is described in U.S. Pat. No. 3,592,637.
More particularly, strontium, either per se or in the form of a strontium-aluminum master composition, is known to be a superior and permanent modifier of the aluminum-silicon eutectic component of eutectic and hypo-eutectic (i.e., less than 12.6 weight percent of silicon) aluminum-silicon casting alloys. Thus, according to Canadian Patent No. 829,816, U.S. Pat. No. 3,446,170, and K. Alker et al., "Experiences with the Permanent Modification of Al-Si Casting Alloys," published in Aluminum, 48(5), 362-367 (1972), the use of alkaline earth metals (e.g., strontium) as modifiers for aluminum-silicon alloys improves the casting properties of the latter, especially when these alloys are at or near the eutectic composition, which is the composition at which the melting point of the alloy is at a minimum. However, the manufacture of such a master composition, containing, for example, about 10 weight percent strontium and 90 percent aluminum, is subject to the disadvantages that the strontium metal added to molten aluminum to form the master alloy is not only expensive but also oxidizes when exposed to air and burns upon fusion in contact with molten aluminum in the ambient atmosphere. A need has therefore existed in the aluminum-silicon casting alloy field for a strontium-containing adjuvant which is free of the aforesaid drawbacks.
Accordingly, it is an object of the present invention to provide an improved strontium-containing master composition for modifying eutectic and hypo-eutectic aluminum-silicon casting alloys.
Another object is to provide a process for producing an improved strontium-containing master composition for modifying the structure of eutectic and hypo-eutectic aluminum-silicon casting alloys.
Yet another object is to provide a process for modifying the structure of eutectic and hypo-eutectic aluminum-silicon casting alloys by the addition thereto of an improved strontium-containing master composition.
These and other objects of the present invention as well as a fuller understanding of the advantages thereof can be had by reference to the following detailed description and claims.